I suggest running the PX-708A self-test diagnostics with a piece of Plextor recommended media, as described in the manual. This should help you determine whether the fault is in the PX-708A, or something else within your system may have malfunctioned. If the self-test fails with Plextor recommended media it's a pretty clear-cut case of drive failure.
Transfer mode switching back to PIO is not a good sign. Other than your PX-708A being suspect, try checking your IDE cables. Also, post some more information about your system: motherboard/chipset names, how your HDD and PX-708A drives are connected to the IDE bus, what CD/DVD burning software are installed, etc.